It is well known that transmitted signals suffer degradation from such factors as noise, inter-symbol interference, and distortion during transmission, and that the extent of such degradation can be represented in an eye closure diagram, which is essentially a plot of signal amplitude against time. For a binary data signal, such a diagram has a single eye which is open or closed to an extent determined by the signal degradation. For optimum regeneration of the signal, it is desired to sample the signal with respect to an amplitude decision or threshold level and at a time positioned optimally within the open part of the eye closure diagram.
Transmission rates of data in communications systems have increased dramatically in recent years, and continue to increase. Additionally, the length of fiber links in networks also continues to increase. Both of these factors can contribute to increased signal degradation that must be compensated for when the transmitted signal is regenerated.
In order to assist in error detection, some standards, for example, those that pertain to Synchronous Data Hierarchy (SDH), have allowed for basic performance monitoring through error detection methods such as Bit-Interleaved Parity-8 (BIP-8), in which parity checks are performed on one-byte groups. More recent standards that have or are being devised for optical networks (for example, ITU-T G.709) provide for overhead that includes extensive forward error correction (FEC) data for use in regeneration of degraded signals.
Various systems have been proposed in which receiver regeneration performance is monitored and the results used in optimization routines in an attempt to lower the Bit Error Rate (BER) in such systems. For example, in U.S. Pat. No. 5,896,391 issued Apr. 20, 1999, to Solheim et al. and assigned to Northern Telecom, forward error correction assisted receiver optimization is disclosed. However, such proposals have focussed on monitoring performance based on the magnitude of the BER, rather than distinguishing between the types of errors that are occurring.
It is therefore desirable to provide a receiver having performance monitoring and correction capabilities in which the types of errors that occur are considered.